для целых чисел
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
<title></title>
<script type="text/javascript" language="javascript">
function setData(input, val)
{
var factor = document.getElementById('factor').innerHTML;
return input.value = parseInt(val)*parseInt(factor);
}
</script>
</head>
<body>
<div id="factor">4</div>
<form method="" action="">
<input onkeyup="setData(this, this.value)" type="text" name="digit"/>
<input type="submit"/>
</form>
</body>
</html>
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